Cracked accounts means that someone has been bruteforcing other people's accounts using their known passwords and shit.
Essentially they are accounts that belong to other people. While I have no experience with cracked OnlyFans accounts, I have bought cracked accounts for shit like VPNs and streaming services to leech off of someone else's accounts. In general they work fine. If you're lucky and careful, they won't notice you're using their accounts. If you're unlucky, they realize right away and you lose your access instantly.
Basically; If you buy one of those "cracked accounts" for OnlyFans, you would be receiving the account of someone else. There's a chance the account does not work and in some cases you might get a replacement.
If you plan on getting a cracked account to subscribe to other people, chances are they'll notice sooner or later and change their OnlyFans password (or OnlyFans might get suspicious and close it prematurely, the latter happened to one of my OnlyFans alt accounts for whatever reason lol).
I guess you might get away with a single $5 subscription, but past that you'll probably get noticed. Some people look at their accounts closely, or even have notifications for each transaction.
Morally speaking, I find it wrong to use someone else's money to subscribe to people (but then again I also leak OnlyFans shit from people that do this as their job, so I'm not gonna say much about that). So there's also that.